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Analysis The UK electricity market has recovered from a troublesome shortage of supply on Tuesday, according to network operator National Grid. However, some analysts are seeing the problems - which included power cuts and reduced supply voltages - as a warning sign of more trouble to come.

National Grid spokesman Stewart Larque described the sequence of events to the Reg yesterday. He said that the start of the problems came a little before midday on Tuesday, when two big power stations went offline unexpectedly, within "two minutes" of each other. Larque didn't specify, but it's now well known that these were the 1,000-megawatt Sizewell B nuclear station in Suffolk and the 2,400-megawatt Longannet coal station in Scotland.

According to Larque, the loss of two big stations so close together caused the frequency of the national grid supply to drop unacceptably as the gap between supply and demand widened. Automatic systems cut in, disconnecting some areas so as to preserve stable supplies for the rest.

At any one time, a fair number of UK powerplants will be sitting idle but ready to fire up. Some types take a long time to come on line, but others are faster - especially gas-turbine stations. Within "forty minutes", according to Larque, sufficient new generation was online to deal with the then level of demand.

However, the Grid prefers to run with a 20 per cent safety margin available in case its demand forecasting doesn't match reality, and there wasn't enough generation available to provide this through the afternoon. At 12:45 the National Grid control centre went straight to its stage-two level of warning to the energy market, a "High Risk of Demand Reduction" notice. (By "demand reduction", the Grid means it may forcibly reduce the amount of power that consumers can use - either by somewhat reducing the voltage supplied to them, or ultimately by cutting them off.)

The initial warning was issued to cover the period until 7pm yesterday, saying that a further 1200 megawatts was required. Another "High risk" warning went out just before 4pm, saying that the gap had now widened to 1600 megawatts - and then eleven minutes later the Grid control room issued the next stage of alert, "Demand Control Imminent".

Shortly after that, "demand control" went into effect, as distribution companies reduced the voltage in their supplies. The effect of this on most electrically powered equipment is to reduce the amount of energy used - in most cases without causing damage or loss of function, there generally being a built-in tolerance to variations in supply.

Larque said that the energy market would normally have been well able to cope without these measures, but apart from Sizewell and Longannet seven other "generating units" had gone offline unexpectedly at various times on Tuesday. (There may be several different generating units at a given power station site, operating as separate entities in the electricity market.) Again, Larque wouldn't give details, but 'leccy-market web data shows that the 250-megawatt Deeside gas plant suddenly shut down at around 3:30pm, just before the Grid control room warnings ramped up and rationing began; and another 80 megawatts had vanished at the South Humber gas plant perhaps twenty minutes before that.

Over the next couple of hours, more machinery either became available or moved to short-notice status, and "Demand Control" rationing ceased as the Grid found it had an operating margin again. At 7:22pm, the "High Risk" warning was cancelled as well.

During Tuesday night, Longannet became operational once more, and the following morning Larque said that it was "business as usual" - though one can't help noticing that the spot electricity price is rocketing upward as of Wednesday evening with no signs of stopping - it's already 50 per cent up on Tuesday's peak.

Evidently the snags at Longannet this time were less serious than in January, when the station had to shut down for some time after the overhead conveyor supplying it with coal collapsed, crushing a shower block (fortunately unoccupied). Scottish politicians rushed through approvals for the station to be allowed to use gas if necessary in future, with one saying they were "lucky it wasn't cold".

As for Sizewell B, a spokesman for British Energy told the Reg today that there had been a "false reading on an instrument panel" which had caused the plant to automatically shut itself down. Sizewell is expected to be back on line "in the next day or so". British Energy don't consider the event to be a technical failure, saying that false readings can never be completely eradicated; the key thing is that the plant should fail safe, which it did in this case.

The National Grid could offer no comment on the situation which had led it to ration power. Larque said that multiple power station shutdowns in one day was far from unknown, but nine in one day was "very unusual".

However, the number of failed stations may be largely irrelevant. It appears that some were sub-100-megawatt units like the South Humber one, fairly insignificant in the context of gigawatt stations like Sizewell and Longannet suddenly vanishing. Even Deeside was small by comparison. When the two large plants - though one notes they are by no means the biggest ones in the UK - unexpectedly went down, it seems that all the spare capacity available was only just enough to compensate, leaving nothing ready to deal with the evening demand surge, despite the Grid's increasingly desperate appeals to the market.

This view would seem to indicate that similar - or more serious - problems could be on the cards in future, as demand is forecast to rise and most new plants in the gigawatt range are facing years of planning disputes and protests from the green movement. (The preferred green solution, offshore wind, likewise faces severe planning difficulties - and it would take three colossal, world's-biggest-ever windfarms like the proposed London Array to deliver the same juice as Sizewell B does, or at least six to match Longannet. And of course, you'd then have problems like Tuesday's every time the wind dropped - or rose to gale force - around most of the British coast, which does happen now and again.)

One energy-market analyst, speaking to the Times, blamed lack of investment owing to years of uncertainty over government policy.

"The Government’s inability to make long-term energy security decisions over the last decade is coming home to roost," said the industry watcher. "Lack of political will to make tough decisions has left Britain short of power."

And of course, all this is only to talk of electricity demand in its current form. If anyone out there fancies electric or hydrogen propelled transport - or would like to stop burning fossil fuels for baths, laundry, heating, industry etc - we're going to need six or seven times as much 'leccy again.

It looks as though we'll be doing pretty well just to keep the lights on. ®